Door latches are used to retain doors, gates and the like in closed positions and, typically, comprise a spring loaded latch bolt which is resiliently urged outwardly of the latch into a latching position in which it is engageable with a retainer or catch on an adjacent door frame to latch the door in its closed position. The latch is retracted in order to permit opening of the door by turning of a square section bar which projects from opposite sides of the latch and the door and which may be turned by handles fitted to opposite ends of the bar. The handles may be knobs or lever arms. If a door having such a latch mechanism is to be locked in its latched position, the latch normally incorporates a separate key-operated bolt or, alternatively, one or both latch handles may be fitted with a complicated central locking mechanism actuated by a central push button or turnable member.
U.S. Pat. No. 1,688,240 comprises a spring loaded latch bolt which is resiliently urged outwardly to engage a catch in the door jamb and which is retractable in response to turning of either of a pair of coaxial latch bars, each fitted with a door knob. To lock the door from the inside, when it is closed, the inside door knob is turned in either direction. This knob is coupled by its latch bar to a cam which, in response to turning of the knob, pushes a locking member mounted on the latch bar, within the latch mechanism, so that a part of the locking member engages a slot in the housing of the mechanism. When engaged with the slot, the locking member prevents the other latch bar from turning so that the outside door knob cannot be turned. Hence, the door cannot be opened from the outside. To release the outside knob, the inside knob must first be turned to retract the latch bolt and this enables the door to be opened. When the door is opened, the inside knob has to be released by the user to enable the spring loaded bolt to extend beyond the door, whereupon the latch bolt pulls the locking member from the slot, thus releasing the outside door knob.
U.S. Pat. No. 5,433,495 also discloses a door latch mechanism within a body of a door, comprising a latch bolt which is retractable in response to turning of a door knob on either side of the door. One of the knobs is designed to be pushed in towards the door to prevent the latch bolt from being retracted by either knob. To enable the door knobs to be able to subsequently retract the latch bolt, the same door knob is then pulled out.
CH-A-565307 discloses a sash window with a crank handle and a spring biased lock.